The present invention is a method for analyzing irregular-shaped chunked silicon for low-level surface and bulk contaminates. The method comprises selecting a zonable chunk of silicon and float-zoning the chunk of silicon to effect a distribution of surface contaminates into the bulk of a monocrystal of silicon. The float-zoned monocrystal of silicon is then processed into a wafer suitable for analysis for low-level contaminates.
Electronic components such as rectifiers, transistors, photo transistors, computer chips, and the like require extremely high purity monocrystalline silicon. In commercial processes this monocrystalline silicon is prepared by first forming polycrystalline silicon ingots by chemical vapor deposition of silanes onto a heated silicon element. These polycrystalline silicon ingots are then converted into monocrystalline silicon by float-zoning or by pulling a monocrystal from a melt, as exemplified by the Czochralski method. The float-zone method is more expensive and is limited by the size of polycrystalline silicon ingot that can be effectively made and float-zoned. Therefore, the Czochralski type process is a commonly practiced commercial process.
In order to be used in the Czochralski type process, the polycrystalline silicon ingots must be broken into chunks of suitable size to be efficiently added to the required melt. The present invention recognizes that during the process of breaking the polycrystalline silicon ingots into chunks and sizing, considerable surface contamination of the silicon can occur. Therefore, to accurately assess the contaminates associated with silicon added to the melt it is necessary to measure both this surface contamination and the bulk contamination of the silicon chunks.
The current most sensitive methods for analyzing silicon for low level of contaminates such as phosphorous and boron require a monocrystalline wafer of silicon. Prior to the present invention this monocrystalline silicon wafer has been prepared by float-zoning of a polycrystalline silicon ingot, as formed or after cutting to a suitable size. The use of such a sample for analyzing for contaminates does not account for contamination imparted to the silicon during the breaking and sizing process. Furthermore, the process of cutting a monocrystalline silicon element into a monocrystalline silicon wafer imparts significant surface contamination to the wafer which must be removed before determining contaminate levels associated with the monocrystalline silicon wafer. Thus, surface contamination cannot be determined.
Therefore, an objective of the present invention is to provide a monocrystalline silicon wafer suitable for analysis for total low-level contaminates such as phosphorous and boron associated with silicon chunks. The inventors have discovered that this objective can be achieved by using a modified float-zoning procedure to float-zone a zonable-chunk of silicon. The float-zoning process effects incorporation of the surface contaminates associated with the silicon chunk into the bulk of the silicon. Therefore during low-level contaminates analysis, the surface contaminates are retained with the sample. By this process the total contaminates, both surface and bulk, associated with silicon chunks suitable for use in a Czochralski type process can be assessed.
Float zoning of silicon rods, as exemplified by British Patent 1,081,827, published Sep. 6, 1967, is well known in the art. Kramer, Solid State Technology, January 1983, page 137, states the commonly assumed dogma of those skilled in the art that the polycrystalline silicon used in float-zoning must have geometrical dimensions with tight tolerances, and a very smooth surface. Therefore, unexpectedly the present inventors have discovered that irregular shaped silicon chunks, as described herein, can be float-zoned to provide monocrystalline silicon suitable for contaminates analysis.
Hwang et al., U.S. Pat. No. 4,912,528, issued Mar. 27, 1990, teaches a process where a silicon rod is float-zoned to provide a sample appropriate for trace element analysis.